*SPOILERS* Ruined it.
If they didn't say "You'll get the bends" or "nitrogen bubbles on the brain" every five minutes, I would have been genuinely shocked when the twist occurred.
Movie was great - better than The Shallows - apart from that fact.
If it is to get a theatrical release, hopefully they remove some of the dialogue, it killed the twist. All that was needed was the quick warning while standing on the boat then moving on to other things she had to watch out for, and the warning from her sister while in the cage "don't swim straight up when you panic, you will die".
By the end, it was so obvious one of them was going to get some side effects that had constantly been mentioned.
I thought maybe I was being harsh, but a friend watched the movie and the moment she heard her sister on the radio after being attacked by a shark, she immediately knew she was hallucinating.